animate between pose/reset position in object data
Rest (not reset) position is the original position that your character is set to, prior to any poses being made. It shouldn't be changed once you've finished creating and rigging your character. 

Pose position allows you to set different poses and save then in a Pose Library or record them as keyframes. You normally start with a starting pose and then save/record as you move it through a series of poses to create the movements you want the character to take over a number of frames. 

Rest position doesn't play any part in your animation. The animation uses only the poses you save/record. 

A simnple experiment is to create an initial pose, keyframe it to frame 1, then set your character to another pose and keyframe it to frame 20. Now press Alt-A to animate the character and watch what happens. Add more poses and keyframe them at various points along the timeline. This will familiarize you to the basic procedure and give you a better idea of how to use poses.

For info on using the Pose Library to save/use poses, consult the documents.